They aren't. They're circles (or they would be, if rain were falling throughout your field of view).

A rainbow appears in a circle of a particular size (the size has to do with the refractive index of water, so it's the same for all rainbows) centered around a point opposite the Sun in the sky (the "antisolar point"). But since the Sun has to be up for a rainbow to appear, the antisolar point is necessarily below the horizon, so less than half the rainbow is visible if you're on a flat surface.

If you're in a position where there can be rain "below" the horizon in your sky (as if, for example, you're in a plane or on top of a high mountain), you can sometimes see the full circle of the rainbow. But most people live on approximately flat surfaces or at the bottom of valleys, not near the tops of steep hills, and it's hard to get the geometry right to have the Sun above you and the rain below, so rainbows are normally just small portions of a circle.